I suspect GAS is the culprit. GAS was expanded to include Outlook Tasks, Notes, and the Address Book at some point last year. To the best of my recollection, my notes first vanished after these three features were added to GAS.

Yes, my vanishing Outlook notes are probably attributable to GAS, but I couldn't disagree more strongly with the CNET posting. Google has done a great job of working most the bugs out of GAS. (Admittedly, it did have its share of bugs a few years ago.) Additionally, Google support has improved markedly over the last few years as well.

Dramatic changes are revolutionizing how we build and use technology. Every company is automating, digitizing, and modernizing operations. We need a better, more connected way to work together as teams so we can harness the insights from our system…

This video shows how to remove a single email address from the Outlook 2010 Auto Suggestion memory.
NOTE: For Outlook 2016 and 2013 perform the exact same steps.
Open a new email: Click the New email button in Outlook.
Start typing the address: …

There are cases when e.g. an IT administrator wants to have full access and view into selected mailboxes on Exchange server, directly from his own email account in Outlook or Outlook Web Access. This proves useful when for example administrator want…